Trussell J, Hatcher RA, Cates W, Stewart FH, Kost K. A Guide to Interpreting Contraceptive Efficacy Studies. Obstetrics and Gynecology 1990; 76:558-67
Mishell DR, Jr. Contraception. New England Journal of Medicine 1989; 320: 777-787
Kimberly D, et al. National Health Statistics Report. Number 86. November 10, 2015